How to ID a modern human

02 May 2012

From a mysterious 13,000-year-old human from West Africa, to ancient genes in modern human DNA, fossil discoveries and new research techniques are making the study of human evolution more exciting than ever.

Homo sapiens skulls are high and rounded with a small brow ridge and bony chin.

But to the untrained eye, one human skull looks a lot like any other. So, how does a scientist distinguish the skeleton of a modern human, Homo sapiens, from other extinct human species such as Homo neanderthalensis, H. erectus and H. heidelbergensis?

There are physical features that, taken together, identify Homo sapiens, says Prof Chris Stringer, human origins expert at the Natural History Museum, who writes on this topic in the journal Nature this week.

'These physical features are found in all humans living today and they had evolved in our African homeland by 100,000 years ago.'

Skull

One of the main features of a modern human is a high and rounded skull. This is visible when viewed from the side, and when viewed from behind, the skull is also wider at the top and narrower at the base.

Homo erectus skulls are long and low and have a strong brow ridge.

In comparison, Homo erectus, who evolved more than 1.5 million years ago, had a skull that was relatively long and low, narrow across the top, and broad across the base.

The size of the skull indicates the brain volume, and modern humans and the Neanderthals have the largest of any of the human species.

Chin

Modern humans have an obvious chin, which is present even in infants. Interestingly, some Neanderthals may show hints of a chin, but this is never found in young Neanderthals.

Brow ridge

A small and divided brow ridge (the area above the eye sockets) is another key feature of modern human skulls. And a narrow area of bone in between the eye sockets.

Homo heidelbergensis skulls show a strong and continuous brow ridge, like all other known ancient human species.

This is unlike, for example, the strong and continuous brow ridge of Homo heidelbergensis, who lived in Africa and Eurasia 500,000 years ago. All of the other known ancient human species have this strong brow ridge.

Skeleton

Modern humans have a lightly built skeleton and a relatively narrow pelvis. All the human species are well adapted to walking upright. Species like the Neanderthals had relatively shorter, heavier and wider bodies, particularly across the shoulders and hips.

Genetics

We all have an evolutionary history written in our genes, and this has shown that more than 90% of our human DNA comes from our recent common African heritage.

But today scientists can also obtain DNA from fossilised bones. In the last couple of years genetic studies have shown that modern humans have a small portion of'archaic' genes.

Homo neanderthalensis skull. Neanderthals were advanced humans sharing features with us and more ancient humans such as H. heidelbergensis.

Outside of Africa they carry about 2.5% of their DNA from Neanderthals, while Australasians also carry about 5% from Denisovans (a newly discovered group known only from a single cave in Siberia so far).

The most likely explanation for this is separate interbreeding events with Neanderthals and Denisovans after modern humans left Africa around 60,000 years ago.

More questions

Did these archaic genes leave any visible traits? And could they explain some of the variation within modern humans, the regional or so-called racial differences?

And if species aren't supposed to interbreed viably with each other, is it time to re-define our species so that this includes other ancient humans? Chris Stringer explores these questions in the Nature article What makes a modern human published this week.